The present invention relates generally to digital to analog converters (DAC) and more particularly to R-2R ladder DACs.
DACs convert a binary input into an analog output voltage. DACs are widely used in disc drive read channels, digital sound and video systems, and many consumer electronics. Modern applications and electronic devices require ever greater resolution to meet the increasing demands of users, such as greater image quality. However, increasing the resolution of a DAC can increase nonlinearities in the output.
For an ideal DAC, every increment of the binary input increases the output voltage by exactly the same amount VLSB. However, real DACs exhibit integral and differential nonlinearities. Integral nonlinearity is the deviation from a line between zero and full scale. Differential non-linearity is a measure of the worst case deviation from the ideal one VLSB step. For example, a DAC with a 1.5 VLSB output change for a 1 least significant bit (LSB) digital code change exhibits 0.5 LSB differential nonlinearity, and a 1 VLSB output change has 0 LSB differential nonlinearity. Differential non-linearity may be expressed in fractional bits or as a percentage of full scale. A differential non-linearity greater than 1 LSB will lead to a non-monotonic transfer function in a DAC. Thus, there would be an undesirable sign change in the slope of the transfer curve.
For many applications, it is beneficial to have a continuous and monotonically increasing voltage output, which may be more important than accuracy. A DAC which is monotonic will be more desirable for applications where the small-signal performance is of importance or possibly where the DAC is in a feedback loop. If a DAC is monotonic, it's output voltage will always increase for increasing values of binary input, and vice versa.
Thus, what is needed are DACs that have relatively small deviations from monotonic behavior and are cost effective to manufacture.